The BRCA1-Δ11q Alternative Splice Isoform Bypasses Germline Mutations and Promotes Therapeutic Resistance to PARP Inhibition and Cisplatin.

Breast and ovarian cancer patients harboring BRCA1/2 germline mutations have clinically benefitted from therapy with PARP inhibitor (PARPi) or platinum compounds, but acquired resistance limits clinical impact. In this study, we investigated the impact of mutations on BRCA1 isoform expression and therapeutic response. Cancer cell lines and tumors harboring mutations in exon 11 of BRCA1 express a BRCA1-Δ11q splice variant lacking the majority of exon 11. The introduction of frameshift mutations to exon 11 resulted in nonsense-mediated mRNA decay of full-length, but not the BRCA1-Δ11q isoform. CRISPR/Cas9 gene editing as well as overexpression experiments revealed that the BRCA1-Δ11q protein was capable of promoting partial PARPi and cisplatin resistance relative to full-length BRCA1, both in vitro and in vivo Furthermore, spliceosome inhibitors reduced BRCA1-Δ11q levels and sensitized cells carrying exon 11 mutations to PARPi treatment. Taken together, our results provided evidence that cancer cells employ a strategy to remove deleterious germline BRCA1 mutations through alternative mRNA splicing, giving rise to isoforms that retain residual activity and contribute to therapeutic resistance. Cancer Res; 76(9); 2778-90. ©2016 AACR

Frat is dispensable for canonical Wnt signaling in mammals

Wnt-signal transduction through β-catenin is thought to require the inhibition of GSK3 by Frat/GBP. To investigate the role of Frat in mammalian development, we have generated mice with targeted mutations in all three murine Frat homologs. We show that Frat is normally expressed at sites of active Wnt signaling. Surprisingly, Frat-deficient mice do not display gross abnormalities. Moreover, canonical Wnt signaling in primary cells is unaffected by the loss of Frat. These studies show that Frat is not an essential component of the canonical Wnt pathway in higher organisms, despite the strict requirement of Frat/GBP for maternal Wnt signaling in Xenopus

Functional Categorization of <i>BRCA1</i> Variants of Uncertain Clinical Significance in Homologous Recombination Repair Complementation Assays

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BRCA1 RING Function Is Essential for Tumor Suppression but Dispensable for Therapy Resistance

SummaryHereditary breast cancers are frequently caused by germline BRCA1 mutations. The BRCA1C61G mutation in the BRCA1 RING domain is a common pathogenic missense variant, which reduces BRCA1/BARD1 heterodimerization and abrogates its ubiquitin ligase activity. To investigate the role of BRCA1 RING function in tumor suppression and therapy response, we introduced the Brca1C61G mutation in a conditional mouse model for BRCA1-associated breast cancer. In contrast to BRCA1-deficient mammary carcinomas, tumors carrying the Brca1C61G mutation responded poorly to platinum drugs and PARP inhibition and rapidly developed resistance while retaining the Brca1C61G mutation. These findings point to hypomorphic activity of the BRCA1-C61G protein that, although unable to prevent tumor development, affects response to therapy

Somatic inactivation of E-cadherin and p53 in mice leads to metastatic lobular mammary carcinoma through induction of anoikis resistance and angiogenesis

Metastatic disease is the primary cause of death in breast cancer, the most common malignancy in Western women. Loss of E-cadherin is associated with tumor metastasis, as well as with invasive lobular carcinoma (ILC), which accounts for 10%-15% of all breast cancers. To study the role of E-cadherin in breast oncogenesis, we have introduced conditional E-cadherin mutations into a mouse tumor model based on epithelium-specific knockout of p53. Combined loss of E-cadherin and p53 resulted in accelerated development of invasive and metastatic mammary carcinomas, which show strong resemblance to human ILC. Moreover, loss of E-cadherin induced anoikis resistance and facilitated angiogenesis, thus promoting metastatic disease. Our results suggest that loss of E-cadherin contributes to both mammary tumor initiation and metastasis

BRCA1(185delAG) tumors may acquire therapy resistance through expression of RING-less BRCA1

markdownabstractHeterozygous germline mutations in breast cancer 1 (BRCA1) strongly predispose women to breast cancer. BRCA1 plays an important role in DNA double-strand break (DSB) repair via homologous recombination (HR), which is important for tumor suppression. Although BRCA1-deficient cells are highly sensitive to treatment with DSB-inducing agents through their HR deficiency (HRD), BRCA1-associated tumors display heterogeneous responses to platinum drugs and poly(ADP-ribose) polymerase (PARP) inhibitors in clinical trials. It is unclear whether all pathogenic BRCA1 mutations have similar effects on the response to therapy. Here, we have investigated mammary tumorigenesis and therapy sensitivity in mice carrying the Brca1 _185stop_ and Brca1 _5382stop_ alleles, which respectively mimic the 2 most common BRCA1 founder mutations, BRCA1 _185delAG_ and BRCA1 _5382insC_. Both the Brca1185stop and Brca1 _5382stop_ mutations predisposed animals to mammary tumors, but Brca1 _185stop_ tumors responded markedly worse to HRD-targeted therapy than did Brca1 _5382stop_ tumors. Mice expressing Brca1 _185stop_ mutations also developed therapy resistance more rapidly than did mice expressing Brca1 _5382stop_. We determined that both murine Brca1 _185stop_ tumors and human BRCA1 _185delAG_ breast cancer cells expressed a really interesting new gene domain-less (RING-less) BRCA1 protein that mediated resistance to HRD-targeted therapies. Together, these results suggest that expression of RING-less BRCA1 may serve as a marker to predict poor response to DSB-inducing therapy in human cancer patients